My buyers aren't able to download on iPad, iPhone or anything Apple. I originally thought it could be the file size as it's over 20MB so sent them free links where all files are under 20MB and they still can't open them. The file contains 4 MP3s and 1 PDF.

The trouble is that Apple iOS devices don't really have a general-purpose file storage system by default; rather, files are associated with specific apps, so if the user doesn't have an app installed that can handle a given file type (such as .zip files), iOS doesn't know what to do with it. Moreover, they'd need to have an app that can decompress (unzip) .zip files to make use of those anyway. I found this article that provides a pretty good overview of the issue and recommends some suitable apps:

All that said, a better approach might be to create separate products for each individual MP3 and PDF file, then edit the main product you're actually selling to enable the "Package files from other products" setting, which will allow you to select each of the products whose files the main product would issue. This would provide buyers with a collection of links to the actual MP3 and PDF files, rather than making them jump through hoops to download and decompress a .zip file.

Once you've got that set up, you may wish to delete the uploaded .zip file to reclaim that storage space. Just re-edit the main product, temporarily checkmark Single File Download to show the Overwrite Product File button, click that button to open a file manager window, click the Delete button there, close that window, and than Back out of the product's settings without submitting any changes.